1. Field of the Invention
The present invention relates to a semiconductor laser apparatus which has extremely stable oscillation characteristics an extremely narrow spectral width at its oscillation output wavelength and is capable of electrically modulating the oscillating output wavelength at an extremely high speed. In particular, the semiconductor laser apparatus of the present invention is useful as a light source for an optical wavelength-division-multiplexing communication system and as a light source suited to coherent communication using coherent light, i.e. a light wave having a uniform spatial phase.
2. Description of the Prior Art
There are a variety of known prior art devices related to conventional semiconductor laser apparatuses either for narrowing the spectral width of oscillating output wavelength, or for stabilizing the oscillation characteristics, or for modulating the oscillating wavelength.
Typically, there is the "distributed feedback semiconductor laser" (DFB laser) which uses a waveguide diffraction grating for forming a light-resonant reflector. Another is the external cavity type semiconductor laser which generates light resonance by applying a diffraction grating and a semiconductor laser. The detail of the DFB laser was reported by M. Nakamura et al. in the technical paper "CW operation of the distributed feedback GaAs-GaAlAs diode lasers at temperature up to 300K", pages 403 through 405 of the Appl. Phys. Lett., 27, 1975. A typical example of the external cavity type semiconductor laser was described by M. Fleming et al. in the technical paper "Spectral Characteristics of External-Cavity Controlled Semiconductor Lasers", pages 44 through 59 of IEEE Journal of Quantum Electronics, QE-17, No. 1, 1981.
FIG. 6 illustrates a structure of a typical conventional DFB semiconductor laser. The reference numeral 601 shown in FIG. 6 designates a laser oscillator; element 602 is a light waveguide; element 603 is a waveguide type diffraction grating; element 604 is an open end surface functioning as a light-resonant reflector at the laser-beam emitting side, and numeral 605 designates a substrate. A light beam is generated by the laser oscillator 601 by the reunion of the electrically injected electrons and positive holes. The light beam enters into the waveguide type diffraction grating 603 via the light waveguide 602. The incident light beam then interacts the diffraction grating, and, only the light beam containing a wavelength having a specific relationship with the period of the diffraction grating is reflected therefrom. The reflected beam returns to the light waveguide, and then passes through the laser oscillator 601 before again being reflected by the conventional reflector 604 formed at the other end surface, thus eventually forming a light resonator together with the waveguide type diffraction grating 603 and the reflector 604. As a result, the resonant wavelength is fixed by the period of the diffraction grating 603, thus eventually generating a stable laser beam having a narrow spectral width of its oscillating output wavelength.
FIG. 7 illustrates a structure of a typical conventional external-cavity type semiconductor laser. The reference numeral 701 shown here designates a semiconductor laser; element 702 is a diffraction grating, and numeral 703 designates a reflector formed on the end surface at the laser-beam emitting side. A light beam generated by the semiconductor laser 701 resonates between the diffraction grating 702 and the reflector 703. However, the resonating wavelength is confined to the beam having a specific wavelength controlled by the diffraction grating, and as a result, a stable laser beam having a narrow spectral width of its oscillating output wavelength is generated.
Nevertheless, none of these conventional semiconductor laser apparatuses have achieved sufficient performance characteristics to be used as the light source of an optical wavelength-division-multiplexing communication system and for use in coherent optical communication. The DFB semiconductor laser uses the waveguide type diffraction grating. However, because of insufficient frequency selectivity, the spectral width of the oscillating output wavelength is not fully narrowed. On the other hand, any conventional external-cavity type semiconductor laser can use a diffraction grating having satisfactory wavelength selectivity. Since the diffraction gratings are apart from each other in space, these gratings are mechanically fixed by matching the optical axes. However, the optical axes cannot easily be matched, and due to varied temperature or displacement caused by mechanical oscillation, even the correctly matched light axes are adversely subjected to thermal expansion or contraction. As a result, actually, it is very difficult for manufacturers to produce stable diffraction gratings. The wider the spatial distance between the diffraction gratings, the narrower the spectral width of the oscillating output wavelength. Conversely, the wider the spatial distance, the easier it is for mechanical oscillation to occur. Consequently, conventional external cavity type semiconductor lasers cannot achieve fully satisfactory performance characteristics today.
A conventional light communication system can perform low-noise communication and wavelength multiplexing. Nevertheless, these conventional semiconductor laser apparatuses cannot perform these operations. This is because of the fact that conventional semiconductor laser apparatus fix the wavelength by applying the diffraction gratings. In order to vary the wavelength, the period and angle against the light of the diffraction grating must also be varied. However, conventional semiconductor laser apparatuses can hardly vary these parameters without applying a mechanical operation. Although the external cavity type semiconductor laser apparatus can vary the oscillating output wavelength by varying the angle of the diffraction grating, conventional systems availing themselves of the mechanical displacement cannot properly modulate frequencies ranging to GH.sub.z at a very high speed.
As is clear from the above description, that conventional semiconductor laser apparatuses cannot electrically and stably modulate frequencies having very narrow spectral width of their oscillating output wavelength at a very high speed in presence of variable surrounding environmental conditions.